Synthesis and evaluation of 18F-labeled benzylguanidine analogs for targeting the human norepinephrine transporter

Abstract

Purpose: Both (131)I- and (123)I-labeled meta-iodobenzylguanidine (MIBG) have been widely used in the clinic for targeted imaging of the norepinephrine transporter (NET). The human NET (hNET) gene has been imaged successfully with (124)I-MIBG positron emission tomography (PET) at time points of >24 h post-injection (p.i.). (18)F-labeled MIBG analogs may be ideal to image hNET expression at time points of <8 h p.i. We developed improved methods for the synthesis of known MIBG analogs, [(18)F]MFBG and [(18)F]PFBG and evaluated them in hNET reporter gene-transduced C6 rat glioma cells and xenografts.

Methods: [(18)F]MFBG and [(18)F]PFBG were synthesized manually using a three-step synthetic scheme. Wild-type and hNET reporter gene-transduced C6 rat glioma cells and xenografts were used to comparatively evaluate the (18)F-labeled analogs with [(123)I]/[(124)I]MIBG.

Results: The fluorination efficacy on benzonitrile was predominantly determined by the position of the trimethylammonium group. The para-isomer afforded higher yields (75 ± 7%) than meta-isomer (21 ± 5%). The reaction of [(18)F]fluorobenzylamine with 1H-pyrazole-1-carboximidamide was more efficient than with 2-methyl-2-thiopseudourea. The overall radiochemical yields (decay-corrected) were 11 ± 2% (n = 12) for [(18)F]MFBG and 41 ± 12% (n = 5) for [(18)F]PFBG, respectively. The specific uptakes of [(18)F]MFBG and [(18)F]PFBG were similar in C6-hNET cells, but 4-fold less than that of [(123)I]/[(124)I]MIBG. However, in vivo [(18)F]MFBG accumulation in C6-hNET tumors was 1.6-fold higher than that of [(18)F]PFBG at 1 h p.i., whereas their uptakes were similar at 4 h. Despite [(18)F]MFBG having a 2.8-fold lower affinity to hNET and approximately 4-fold lower cell uptake in vitro compared to [(123)I]/[(124)I]MIBG, PET imaging demonstrated that [(18)F]MFBG was able to visualize C6-hNET xenografts better than [(124)I]MIBG. Biodistribution studies showed [(18)F]MFBG and (123)I-MIBG had a similar tumor accumulation, which was lower than that of no-carrier-added [(124)I]MIBG, but [(18)F]MFBG showed a significantly more rapid body clearance and lower uptake in most non-targeting organs.

Conclusion: [(18)F]MFBG and [(18)F]PFBG were synthesized in reasonable radiochemical yields under milder conditions. [(18)F]MFBG is a better PET ligand to image hNET expression in vivo at 1-4 h p.i. than both [(18)F]PFBG and [(123)I]/[(124)I]MIBG.

Figure 1

Synthetic scheme for radiolabeling benzylguanidine analogs ([¹⁸F]MFBG).

Figure 2

In vitro uptake of [¹⁸F]MFBG, [¹⁸F]PFBG, ¹²³I-MIBG and [¹²⁴I]MIBG in the hNET-transduced and wild-type C6 cells after 2 h incubation at 37 °C. [¹⁸F]MFBG and [¹⁸F]PFBG showed a similar uptake (P = 0.94), which was 4-fold lower than that of ¹²³I/[¹²⁴I]MIBG (P < 0.01). MIBG (200 µM) was used in the blocking experiments. The results (mean ± SD) are from 2–5 independent studies with triplicates in each experiment.

Figure 3

In vivo accumulation of [¹⁸F]MFBG, [¹⁸F]PFBG and [¹²⁴I]MIBG in animals bearing dual xenografts (C6-hNET (right) and C6-WT (left)). (A) and (B) PET images (coronal) and ROI analysis of [¹⁸F]MFBG and [¹⁸F]PFBG accumulation at 1 h and 4 h post injection; (C) PET images (coronal) and ROI analysis of [¹²⁴I]MIBG accumulation at 1 h, 4 h and 24 h post injection. The maximum pixel value (%ID/mL) was utilized to diminish partial volume effects. The results (mean ± SD) of ROI analysis are from groups of seven ([¹⁸F]MFBG) or five ([¹⁸F]PFBG and [¹²⁴I]MIBG) animals each. [¹⁸F]MFBG visualized C6-hNET xenografts with a higher tumor-to-background ratio.

Figure 4

Typical PET projection images of [¹⁸F]MFBG, [¹⁸F]PFBG and [¹²⁴I]MIBG in animals bearing dual xenografts (C6-hNET (right) and C6-WT (left)). [¹⁸F]MFBG (A) and [¹⁸F]PFBG (B) PET imaged at 1 h and 4 h p.i., respectively; (C) PET images of [¹²⁴I]MIBG at 1 h, 4 h and 24 h post injection. The projection images are from the same animals shown in Figure 3. A color threshold was optimized to visualize the C6-hNET tumor (white arrow) on the projection image; the C6 wild-type tumor (pink arrow) was not visualized.